The B-WELL-Mom study aims to increase understanding of factors that predict poor asthma control during pregnancy as well as add to our knowledge of the basic immunology of pregnancy. Asthma is a common chronic disease and some women experience exacerbation and worsening of their asthma during pregnancy while others improve. Change in asthma severity/control in pregnancy has been noted frequently in the literature (about one-third worsen and one-third improve) but this change is not explored in any depth1. Patients with asthma may have a defect in regulatory T (Treg) cells, which favors further TH2-cell proliferation2. Treg cells are integral to the process by which the immune system differentiates foreign antigens (pathogens, etc.) and self antigens and, as such, they also play a key role in mediating pregnancy tolerance3. Treg can be measured in peripheral blood and the levels vary over the course of pregnancy and post-partum as well as across the menstrual cycle. Foxp3 is the primary gene responsible for Treg regulation. B-WELL-Mom investigates predictors of asthma control variability during pregnancy including demographic, biologic, genetic and environmental factors with particular interest in the role of maternal allergy. Pregnancy requires the immune system to tolerate the ?not self? fetus and the clinical presentation of asthma including improvement or worsening will contribute to knowledge of the basic immunology of pregnancy. Women who get worse during pregnancy may represent a vulnerable subgroup with respect to environmental exposures, such as poor air quality, which can exacerbate asthma and may have an independent effect on poor pregnancy outcomes.